@dragon1412 But it was around 10 years ago that it really hit it off with High School DxD's success with it. It's like how OreImo helped popularize the incest trope in shonen (before, it was rare and only with older series; though, ironically, the author never meant it to be that way and wanted the surprise "you aren't blood related" twist that peaked in popularity when the series came out).
And even then, for older series, while there was drama, the harem element over all could leave you unsatisfied. And as time goes on, you could change and who you'd wish won could change.
So know, I'm disliking half assed harems, semi-harems which then introduce the girl that might have liked the MC only to add another love interest. I also hate when the side character girl is made over sexy since then it just feels weird and wrong and makes you wish the MC was with her instead...
And I'm not saying that these series have to be bad. Nisekoi and a few others were actually quite good and fun... but then you feel like it's wrong no matter who wins, since if the girls move on, it feels like a betrayal, but when they get with nobody, you feel too bad for them...